Interference detection method and an interference avoidance method

ABSTRACT

In a radio communication system in which multiple access is carried out between a base station and a plurality of subscriber stations by a TDMA communication method, occurrence of the interference in the R channels through which the subscriber stations ( 21  to  24 ) issue a call request to the base station ( 1 ) can be detected precisely, and time slot arrangement of the R channels is changed by detecting the interference to thus avoid the interference.

TECHNICAL FIELD

The present invention relates to a method for detecting interferenceoccurred between service areas, etc. in the TDMA radio communication anda method for avoiding the interference.

BACKGROUND ART

In the radio communication system in which a fixed number of frequenciesare assigned and used repeatedly to service areas, which are arranged tocover continuously the surface, to carry out the multiple access betweenone base station provided in each service area and a plurality ofsubscriber stations by virtue of the TDMA method, since the serviceareas which employ the same frequency as that of user's own station arearranged at a distance, interference of radio waves which occurs betweenthe service areas utilizing the same frequency must be prevented.

As an example of a means for preventing the interference between theservice areas, the technology for tilting downward the antennadirectivity of the base station is described in “Foundation of MobileCommunication” Chapter 10, Paragraph 10.2.5, supervised by YoshihisaOkumura and Masaaki Shinji and published by the Institute ofElectronics, Information and Communication Engineers, 1986.

FIG. 1 is a diagram showing a method which avoids the interference bysplitting the service area into six sectors and then switching theantenna whose horizontal-plane directivity is narrowed (referred to as a“sector antenna” hereinafter) to respective sectors.

More particularly, a base station 1 can avoid the situation that aninterference wave 51 incoming from other service area 921 which islocated on the opposite side to the subscriber station is received, byusing a third sector antenna 113 to communicate with a subscriberstation 21 and using a fourth sector antenna 114 to communicate with asubscriber station 22.

Also, as for the down multiple address channel (Bch) used to transmitthe message from the base station 1 to the subscriber stations 21, 22,the transmitting sector antenna is controlled to be switchedsequentially, such as an antenna 111 in a TDMA frame 31, an antenna 112in a TDMA frame 32, etc. Therefore, each of the subscriber stations 21,22 can receive surely the multiple address channel once in the period ofTDMA super frame 3. Similarly, as for the up access request channel(Rch) used to transmit the message from the subscriber stations 21, 22to the base station 1, the subscriber stations 21, 22 can receive the upaccess request channel once in the period of TDMA super frame 3 withoutfail.

In the meanwhile, as an example of the method which detects and avoidspositively the interference, procedures are decided in thesecond-generation cordless telephone system (PHS) such that a basestation (CS) and a mobile station (PS) measure the FER (frame errorrate) of the received signal respectively, then recognizes thatinterference occurs in the concerned communication channel if the FERexceeds a predetermined value, and then carries out an interferenceavoidance operation in the order of switching the channel to otherchannel on the same carrier, switching the channel to a channel on othercarrier, switching the channel to a channel of other base station,automatic reconnection, temporary stop of transmission, and release ofthe radio channel, in response to the level of FER. These channelswitching operations are carried out when the base station issues theinstruction to the mobile station via a control channel. Also, ifinterference is detected by the mobile station, the channel switching iscarried out when the mobile station issues the channel switching requestto the base station.

FIG. 2 is a diagram showing an example of a TDMA frame format in suchradio communication system. A TDMA frame 30 consists of four kinds ofTDM channels 40 each having the different purpose, e.g., B channel 41, Rchannel 42, Uu channels 431 to 43 n, and Ud channels 441 to 43 n.

The B channel 41 is the multiple address channel directed from the basestation to the subscriber station, and used to inform the subscriberstation of a frame timing synchronization and control information commonto the subscriber stations. The R channel 42 is a channel used to issuethe connection request from the subscriber station to the base station,and has normally a method to which respective subscriber stations canaccess at random. Accordingly, the collision of communication burstsoccurs due to random access in the R channel. However, in order toreduce the collision occurring probability, as shown in FIG. 3, thereexists the system in which a plurality of R channels (R1) 421 to (R4)424 are arranged in one TDMA frame 30. Uu's are up user channels whichare set one by one between the subscriber stations and the base station,and Ud's are similar down user channels. Each user channel consists of apreamble (PR) 61, an individual control information time slot (C) 62,and time slots (D) 631 to 63 n having the number which corresponds to anamount of information.

In addition, in the method which measures an error rate of the receivedsignal to detect the interference and then performs re-transmission ofthe interfered packet, change of the communication channel, etc., if therandom access type channel such as the R channel is present, it isdifficult to distinguish the interference from the collision because thecollision of the transmitting bursts is assumed as the premise in suchchannel. In order to overcome this problem, based on the event theinterference wave is not synchronized with the transmitting burst, anapproach may be utilized which decides the receiving burst as thecollision if such receiving burst lacks in all time slots and decidesthe receiving burst as the interference if such receiving burst lackspartially.

However, in the radio communication system which performs the multipleaccess between one base station and a plurality of subscriber stationsby virtue of the TDMA method, if the surface service is intended toachieve, the avoidance of the interference only by using the antennadirectivity is limited because of the topographical complexity of theservice areas, etc. In addition, the system typified by the PHS, inwhich both the base station and the subscriber stations makearrangements to avoid the interference, is effective as the system whichavoids the interference in the user channels, but such system cannoteffectively function in the channel, which is used commonly by aplurality of subscriber stations, such as the multiple access channel,the random access type channel, etc.

Further, in the system typified by the subscriber radio access system,which has a large transmission capacitance per channel and in which theburst length of the user channel is varied in response to the service,there are the cases where the burst length of the interference wave islonger several times than the random access channel. Therefore, thissystem contains such a problem that, if all time slots of the randomaccess channel are left out, it is impossible to distinguish theinterference from the collision.

Moreover, in the case of the communication system in which the basestation and the subscriber stations are fixedly operated, since both theinterfering station and the interfered station are not moved, theinfluence of the interference lasts for a long time once they areaffected by the interference.

As a result, the avoidance of the interference onto the multiple accesschannel and the random access channel is difficult, and the particularsubscriber stations get into the situation that they cannot be connectedfor a long time once the interference occurs.

The present invention has been made in view of the above problems, andit is an object of the present invention to provide a method fordetecting precisely occurrence of the interference in the TDMA radiocommunication and provide a method for avoiding effectively theinterference.

DISCLOSURE OF THE INVENTION

The present invention provides an interference detection method for aradio communication system in which multiple access is carried outbetween a base station and a plurality of subscriber stations by a TDMAmethod and a plurality of random access channels are assigned to oneframe of the TDMA method, wherein it is decided that, when areception-unable detection rate in a random access channel is increasedlarger than reception-unable detection rates in other random accesschannels by more than a predetermined value, interference occurs in therandom access channel. Therefore, it is possible to discriminate thecollision from the interference on the channel, and thus only theinterference can be detected precisely.

The present invention provides an interference detection method for aradio communication system in which multiple access is carried outbetween a base station which has a sector switching type antenna and aplurality of subscriber stations by a TDMA method and one random accesschannel is assigned to one frame of the TDMA method, wherein it isdecided that, when a reception-unable detection rate in a random accesschannel is increased larger than reception-unable detection rates inother random access channels by more than a predetermined value,interference occurs in the random access channel in the sector.Therefore, it is possible to discriminate the collision from theinterference on the channel, and thus only the interference can bedetected precisely.

The present invention provides an interference avoidance method for aradio communication system in which multiple access is carried outbetween a base station and a plurality of subscriber stations by using aTDMA method, wherein, when interference is detected from a random accesschannel for transmitting from a subscriber station to the base station,the base station closes the random access channel, changes arrangementof the channel to another time slot, and then informs the subscriberstations of close of the random access channel and time slot arrangementchange information via multiple access channels. Therefore, thesituation that the subscriber stations are brought into the accessunable state to the base station can be prevented, and the interferencecan be avoided.

In the present invention, the base station listens the closed randomaccess channel for a predetermined time to detect whether or not aninterference wave is present. Therefore, continuation of theinterference can be checked and thus the effective interferenceavoidance method can be adopted.

The present invention provides an interference avoidance method for aradio communication system in which multiple access is carried outbetween a base station and a plurality of subscriber stations by using aTDMA method, wherein the base station changes time slot arrangements ofmultiple access channels for transmitting to the subscriber stations andrandom access channels for receiving from the subscriber stations everypredetermined frames, and informs the subscriber stations of time slotarrangement changing information via the multiple access channels whichare selected prior to change of frames by at least one frame. Therefore,such a situation can be avoided that the random access channels in whichthe interference is difficult to detect do not function for a long timebecause of the interference disturbance.

In the present invention, a plurality of multiple access channels orrandom access channels are arranged in one frame, and time slotarrangement of at least one channel out of the multiple access channelsor the random access channels is changed every predetermined frames.Therefore, such a situation can be avoided that the multiple accesschannels or the random access channels in which the interference isdifficult to detect do not function for a long time because of theinterference disturbance.

In the present invention, the base station changes time slot arrangementof the multiple access channels or the random access channels based on apreviously decided pattern, and informs the subscriber stations ofcurrent information on the pattern in which the time slot arrangement ischanged via the multiple access channels. Therefore, such a situationcan be avoided that the multiple access channels or the random accesschannels in which the interference is difficult to detect do notfunction for a long time because of the interference disturbance. Inaddition, the probability that the subscriber stations miss positions ofthe multiple access channels can be lowered by using the fixed pattern.

The present invention provides an interference avoidance method for aradio communication system in which multiple access is carried outbetween a base station which has a sector switching type antenna and aplurality of subscriber stations by a TDMA method and one multipleaccess channel and one random access channel are assigned to one frameof the TDMA method, wherein the base station changes sector switchingprocedures in transmission/reception of these channels based on apreviously decided pattern every super frame, and also informs thesubscriber stations of current information on the pattern in whichsector switching is changed via the multiple access channels. Therefore,disturbance of the multiple access channels or the random accesschannels caused by the interference wave, which occurs periodically inthe particular sector, for a long time can be prevented.

The present invention provides an interference avoidance method for aradio communication system in which multiple access is carried outbetween a base station which has a sector switching type antenna and aplurality of subscriber stations by a TDMA method and one multipleaccess channel and one random access channel are assigned to one frameof the TDMA method, wherein the base station changes sector switchingprocedures in transmission/reception of these channels every superframe, and also informs the subscriber stations of the sector switchingprocedures after sector switching is changed via the multiple accesschannels which are selected prior to change of procedures by at leastone frame. Therefore, disturbance of the multiple access channels or therandom access channels caused by the interference wave, which occursperiodically in the particular sector, for a long time can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a method for avoiding the interference byswitching a sector antenna;

FIG. 2 is a frame configuration diagram showing a TDMA frame format;

FIG. 3 is a frame configuration diagram showing another TDMA frameformat;

FIG. 4 is a diagram showing a peripheral configuration of a base stationwhich has sector switching type antennas according to an embodiment 1 ofthe present invention;

FIG. 5 is a diagram showing an operation according to the embodiment 1of the present invention;

FIG. 6 is a characteristic diagram showing interference detection ratesevery sector;

FIG. 7 is a frame configuration diagram showing a TDMA frame formataccording to an embodiment 2 of the present invention;

FIG. 8 is a characteristic diagram showing interference detection ratesevery sector;

FIG. 9 is a diagram showing an operation of an interference avoidancemethod according to an embodiment 3 of the present invention;

FIG. 10 is a frame configuration diagram showing a TDMA frame format ofan interference avoidance method according to an embodiment 4 of thepresent invention;

FIG. 11 is a frame configuration diagram showing a TDMA frame format ofan interference avoidance method according to an embodiment 5 of thepresent invention; and

FIG. 12 is a diagram showing the interference avoidance method accordingto the embodiment 5 of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In order to explain the present invention in more detail, best modes forcarrying out the present invention will be explained with reference tothe accompanying drawings hereinafter.

Embodiment 1

An embodiment 1 of the present invention will be explained withreference to FIG. 1, FIG. 4, and FIG. 5 hereunder. In FIG. 4, assumethat subscriber stations 21 to 24, which are accommodated in serviceareas 911 to 916 of a base station 1, issue an access request to thebase station 1, which is equipped with sector switching type antennas111 to 116, via R channels 71 to 74 respectively. Since the R channelsconsist of a random access channel, the collision of channels occur ifaccess timing coincide with each other between the subscriber stations22 and 23 in the sector 913, for example, and thus the interference isdetected in the time slot by the base station 1.

Meanwhile, the radio wave 51 transmitted from a remote service area 921interferes with the radio wave on an up channel 71 of the subscriberstation 21. As a result, the base station 1 cannot receive the radiowave and, like the above, the interference is also detected in theconcerned time slot.

The base station 1 inputs the interference detection number in the timeslots, which are assigned to the R channel, into an interference timeslot database/interference decision processing section 101 every sector911 to 916. This interference decision processing section 101 calculatesa value which is derived by dividing the interference detection numberby the R channel receiving number of times (interference detection rate)every sector 911 to 916. In this case, the R channel receiving number oftimes is counted up by one when a value of the received electric fieldstrength signal (RSSI) in the base station 1 exceeds a predeterminedvalue in all time slots being assigned to the R channel.

FIG. 6 shows examples of interference detection rates every sector 911to 916, which are calculated by the interference decision processingsection 101. Since the R channel is of the random access type asdescribed above, it is impossible to avoid the interference occurredalways at a constant rate. Therefore, in the sectors 911 to 915 in whichno interference wave 51 exists, the interference detection rateindicates a predetermined rate which corresponds to the channel numberbeing accommodated in the sectors 911 to 915.

In contrast, since the influence of the interference wave is causedperiodically over a plurality of frames unlike the collision of the Rchannel bursts, as shown in FIG. 6, the interference detection rate isincreased in the sector 916, in which the interference wave 51 ispresent, rather than other sectors. The interference decision processingsection 101 detects the sector 916 which has the larger interferencedetection rate than those in other sectors 911 to 915, and then decidesthat the interference has occurred in the concerned sector 916.

FIG. 5 is a diagram showing an example of an interference avoidanceoperation when the interference is detected in the R channel 42 by theabove interference detecting method.

More particularly, in the TDMA frame 30, when the subscriber station 21transmits the call request 82 to the base station 1 via the R channel 42in the same TDMA frame 30 based on a head time slot positionnotification 81 transmitted from the base station 1 via the B channel41, the interference wave 51 occurs and thus the call request 82 issuedfrom the subscriber station 21 cannot be received by the base station 1.In this TDMA frame 30, when the base station 1 detects thisinterference, it sets another R channel 42 b in the time slot withoutthe interference. In the succeeding TDMA frame 31, the base station 1transmits the close of the time slot on which the R channel 42 isarranged and the head time slot position notification 83 of the newlyassigned R channel 42 b via the B channel 41.

The subscriber station 21, when receives this notification 83, transmitsthe call re-request 84 via the R channel 42 b of the newly assigned timeslot. Since the R channel 42 b has no influence of the interference wave51, the base station 1 can accept the call request issued by thesubscriber station 21. In the succeeding TDMA frame 32, the base station1 transmits the head time slot position notification of the R channel aswell as the channel assignment information 85 for the subscriber station21 via the B channel.

Then, according to the close notification 83 of the R channel 42, allsubscriber stations which are present in the service area of the basestation 1 can stop all accesses in the closed time slot 42.

The base station 1 listens the closed time slot 42 to check the presenceof the interference wave 51. Then, if the base station 1 can check thedisappearance of the interference wave for a constant period or more inthe concerned time slot, the R channel 42 b is arranged again at theoriginal time slot 42. In this case, the base station 1 may assignanother channel to the concerned time slot 42, in place of rearrangingthe R channel 42 b into the original time slot 42.

Embodiment 2

An embodiment 2 of the present invention will be explained withreference to FIG. 7 and FIG. 8 hereunder.

In FIG. 7, references 30, 31, 32 denote an example of the TDMA frame inthe radio base station respectively. In this TDMA frame, first to fourthR channels (referred to as “R1 to R4 channels” respectively hereinafter)422 to 424 are arranged to be distributed between the up user channelsU_(U1), . . . , U_(UK-1), U_(UK), . . . , U_(UN) and the down userchannels U_(D1), . . . , U_(DK-1), U_(DK), . . . , U_(DN) respectively.The head time slot positions of the R channels are informed to thesubscriber stations 21 to 23 via the B channel 41. Now, apart of the endportion of the up user channels U_(U1), . . . , U_(UK-1) to a part ofthe head portion of the R2 channel 422 in the base station receives theinterference 51 from other station. In the TDMA frame 31, the accessrequest 82 from the subscriber station 21 and the access request 83 fromthe subscriber station 22 collides with each other on the R1 channel 421to make the reception impossible.

The base station generates an interference detection database every timeslot assigned to each R channel, and calculates a running average of theinterference detection rate over a predetermined frame period. FIG. 8shows the running average of the interference detection rates of timeslots 101 to 106 constituting the R1 channel 421 and the R2 channel 422,which are calculated by the base station, relative to the time.

In the TDMA frame 31, since the collision occurs in the R1 channel 421,the running average values of the time slots 101 to 103 are increased.However, since the collision of the access requests is not continuedover a plurality of frames, such collision never exceeds the collisionoccurring probability which can be previously set.

In contrast, the time slot 104 of the R2 channel 422, which is subjectedto the interference of the other station, exhibits the high interferencedetection rate since the receiving unable state is continued over aplurality of frames.

The base station calculates an average value of the interferencedetection rates of the time slots assigned to all R channels everyframe. Then, the base station decides that the interference occurs inthe time slots in which the interference is detected at a predeterminedrate in excess of this value and the collision occurs in the remainingtime slots.

The avoidance method applied when the interference is detected has beenexplained in the above embodiment 1. As the threshold value for decidingthe interference and the collision, a difference from the average valueof all time slots may be employed like this method, or the collisionoccurring probability which is set previously at the time of channeldesign may be employed as a reference.

Embodiment 3

FIG. 9 is a diagram showing an embodiment 3 of the present invention.The first to fourth R channels 421 to 424 are arranged in the TDMAframes 30 to 34 of the embodiment 3 respectively, and the head time slotpositions of the R channels 421 to 424 are informed to the subscriberstations 21 to 23 via B channels 41 positioned at the head of respectiveframes.

Here, assume that, in the TDMA frame 30, the base station 1 transmitsthe head time slot position notification 80 of the R channels 421 to 424to the subscriber stations 21 to 23 via the B channel 41 and then thesubscriber station 23 transmits the call request 81 to the base station1 via the R1 channel 421 and also the subscriber station 22 transmitsthe call request 82 to the base station 1 via the R2 channel 422. Thecall request 82 issued from the subscriber station 22 is accepted by thebase station 1, but the call request 81 issued from the subscriberstation 23 is not accepted because the interference 51 occurs in the R1channel 421. At this time, the base station 1 has detected theinterference by the means described in the above embodiment 1 or 2, andthen broadcasts the notification 83 via the B channel 41 to thesubscriber stations 21 to 23 in the succeeding TDMA frame 31 to informthe close of the R1 channel together with the head time slot positionnotification of the R2 to R4 channels and the channel assignmentinformation for the subscriber station 22. The subscriber station 23,when receives this notification 83, interrupts the call request via theR1 channel 421 and then issues the call request 85 via the R1 channel423. Also, in the TDMA frame 31, the subscriber station 21 issues thecall request 84 via the R2 channel. These call requests 84, 85 issuedfrom the subscriber stations 21, 23 are accepted by the base station 1.In the TDMA frame 32, the channel assignment information 86 togetherwith the R1 channel close/R2 to R4 channel head time slot positionnotification is transmitted to the subscriber stations 21 to 23 via theB channel. In this case, the subscriber station 23 selects the R3channel as another channel after it has interrupted the call request viathe R1 channel, but this selecting procedure may conducted in the orderof channel number or may conducted at random.

When all subscriber stations receive the R1 channel close notificationvia the B channel 41 in the TDMA frame 31, all subscriber stations whichare located in the service area of the base station 1 stop the access tothe R1 channel to thus complete the close of the concerned channel. Incontrast, the base station 1 listens respective time slots in the closedR1 channel and also checks the presence of the interference wave for apredetermined time. After this, the base station 1 checks the same timeslot periodically, then releases the close of the R1 channel 421 if theinterference wave cannot be detected by a predetermined number of time,and then informs respective subscriber stations of the head time slotposition information 87 of the R1 to R4 channels via the B channel 41 inthe succeeding TDMA frame 33. Then, the base station 1 releases thenotification of the close of the R1 channel 421.

In this case, like this embodiment, the close and check of the Rchannels 421 to 424 may be executed after the interference has beendetected, otherwise the base station 1 may execute such close and checkperiodically.

Embodiment 4

FIG. 10 is a frame configuration diagram showing an embodiment 4 of thepresent invention. In FIG. 10, 30 to 33 denote a TDMA frame in the radiobase station respectively. The TDMA frames 30 to 33 have one R channel42 which is arranged at any position between the up user channelsU_(U1), . . . , U_(UN) or the down user channels U_(D1), . . . , U_(DN),respectively.

In FIG. 10, the base station is subjected to the interference 51 fromother station, so that the R channel 42 a is brought into its unablestate in the TDMA frame 30.

In the embodiment 4, the time slot positions of the R channel 42 arechanged every frame, such as 42 a in the frame 30, 42 b in the frame 31,42 c in the frame 32, and 42 d in the frame 33. Accordingly, as shown inFIG. 10, the R channels 42 a, 42 b cannot be used in the TDMA frames 30,31 because of the influence of the interference wave 51, while theinfluence of the interference can be avoided in the TDMA frames 32, 33.

The time slot arrangement of the R channels, which is changed frame byframe, may be selected in accordance with the previously set pattern orthe random pattern which is decided arbitrarily by the base station. Inaddition, the time slot whose interference is detected during itsassignment as the user channel may be controlled such that succeedingassignment of the R channel may not be applied to such time slot.

The base station can prevent that the subscriber stations lose theposition of the R channel as the result of above control, by informingsimultaneously the head time slot position of the R channel in thecurrent frame and the head time slot position of the R channel in atleast the succeeding frame via the B channel 41 every frame.

Like the R channel 42, the arrangement of the B channel 41 may also bechanged every TDMA frame. In this case, the base station transmits thehead time slot position information of its own channel and the B channeltransmitting position information at least in the succeeding frame inaddition to the position information of the R channel to each subscriberstation via the B channel 41.

Further, time slot arrangement changing timings of the B channel 41 andthe R channel 42 may be switched every super frame in the system whichemploys the sector switching type antenna, etc.

Embodiment 5

FIG. 11 is a frame configuration diagram showing an embodiment 5 of thepresent invention. In FIG. 11, 30 to 32 denote a TDMA frame in thesubscriber station respectively. The TDMA frames 30 to 32 have two Bchannels 411, 412 and two R channels 421, 422 respectively. The Bchannel 412 and the R channel 422 are arranged at any position betweenthe up user channels U_(U1), . . . , U_(UN) or the down user channelsU_(D1), . . . , U_(DN). In FIG. 11, all time slots of the up and downuser channels are depicted for convenience. However, the individualsubscriber station can communicate by using several time slots of theseuser channel group, which are assigned by the base station. In FIG. 11,this subscriber station now receives the interferences 51, 52 from aplurality of other stations, and the interference 51 interferes with thefirst B channel 411 and thus the frame synchronization according to thechannel becomes impossible.

In the embodiment 5, since a plurality of B channels are arranged in oneTDMA frame, normally the subscriber station can maintain thesynchronization with the base station by receiving the second B channel412 even if one of the B channels is brought into its receiving unablestate by the interference 51 for a long time. However, in the case ofFIG. 11, since both the first and second B channels 411, 412 receivesthe interference by the interference waves 51, 52 in the TDMA frame 30,it is impossible for the subscriber station to maintain thesynchronization with the base station in this TDMA frame 30.

In the embodiment 5, the time slot positions of the B channel 412 arechanged every frame, such as 412 a in the frame 30, 412 b in the frame31, and 412 c in the frame 32. Accordingly, the B channels 411, 412 acannot be used in the TDMA frame 30 because of the influence of theinterference waves 51, 52, but the position-variable B channels 412 b,412 c can be received in the TDMA frames 31, 32. Therefore, thesubscriber station can maintain the synchronization with the basestation.

The time slot arrangement of the B channels whose position can bechanged frame by frame may be selected in compliance with the previouslyset pattern or the random pattern which is arbitrarily decided by thebase station. In addition, the time slot whose interference is detectedduring its assignment as the user channel may be controlled such thatsucceeding assignment of the R channel may not be applied to such timeslot.

The base station can prevent that the subscriber stations lose theposition of the B channel as the result of above control, by informingthe head time slot position of the B channel in the current frame andthe head time slot position of the position-variable B channel in atleast the succeeding frame at the same time via all B channels 411, 412every frame.

Like the B channels 411, 412, as shown in FIG. 11, the arrangement ofthe R channels 421, 422 may also be changed every TDMA frame. In thiscase, the base station informs the subscriber stations of the head timeslot position of the B channel which is now transmitting, the head timeslot position of the position-variable B channel at least in thesucceeding frame, the head time slot position information of the firstand second R channels 421, 422, and the head time slot positioninformation of the first and second R channels 421, 422 at least in thesucceeding frame via the B channels 411, 412. Further, time slotarrangement changing timings of the first and second B channels 41 andthe R channels 421, 422 may be switched every super frame in the systemwhich employs the sector switching type antenna, etc.

Embodiment 6

FIG. 12 is a diagram showing an embodiment 6 of the present invention.The base station 1 communicates with the subscribers 21, 22 located inthe service area of own station by using the type beam antennas 111 to116. In FIG. 12, 3001 denotes shift of the frame number, 3002 denotesthe direction of the antenna beam in the B channel in each frame (sectornumber), 3003 denotes the direction of the antenna beam in the R channel(sector number), and 3004 denotes the direction of the antenna beam inthe user channel.

In the system shown in FIG. 12, the antenna beam directions in the Bchannels serving as the multiple access channel and the R channelsserving as the random access channel are switched every frame, andperiods in which the antenna beam takes a round of the service area,i.e., six frame periods are defined as one super frame 300 to 302.

The subscriber station which intends to issue the access request to thebase station 1 checks the received content of the B channel, and thenaccesses the R channel in the succeeding frame. Therefore, the antennabeam direction of the R channel is controlled to be delayed from the Bchannel by one frame. Also, the antenna beam switching direction of theB channel is shifted frame by frame every super frame, e.g., thedirection is started from the sector 1 in the super frame 300, thedirection is started from the sector 2 in the super frame 301, thedirection is started from the sector 3 in the super frame 302, and thelike.

Here, the case is assumed where the subscriber station 21 receives theinterference caused by the interference wave 51 from other station.

For the interference between the same communication systems, the sectorswitching type antennas are also mounted on the base station in theperipheral service area. Therefore, it may considered that theinterference between the B channels or the R channels is caused in thesuper frame period, as shown in FIG. 12.

In FIG. 12, the interference wave 51 is generated in the third frame ofthe super frame. Thus, in the super frame 301 in which the subscriberstation 21 receives the B channel via the this frame, the subscriberstation 21 cannot receive the B channel due to the influence of theinterference wave 51. However, as described above, since the antennabeam switching direction is shifted every super frame, there is nosituation that the influence of the interference wave 51 on thesubscriber station 21 carries on.

1. An interference avoidance method for a radio communication system inwhich multiple access is carried out between a base station and aplurality of subscriber stations by using a TDMA system, comprising thesteps of changing time slot arrangements of multiple access channels fortransmitting signals to the subscriber stations and random accesschannels for receiving signals from the subscriber stations at apredetermined TDMA frame interval, and informing the subscriber stationsof time slot arrangement changing information via the multiple accesschannels which are selected prior to change of frames by at least oneframe interval.
 2. The interference avoidance method according to claim1, wherein a plurality of multiple access channels or random accesschannels are arranged in one frame, and time slot arrangement of atleast one channel out of said plurality of multiple access channels orthe random access channels is changed at a predetermined TDMA frameinterval.
 3. The interference avoidance method according to claim 1,wherein the base station changes time slot arrangements of the multipleaccess channels or the random access channels based on a previouslydecided pattern, and informs the subscriber stations of currentinformation on the pattern in which the time slot arrangement is changedvia the multiple access channels.
 4. An interference avoidance methodfor a radio communication system in which multiple access is carried outbetween a base station which has a sector switching type antenna and aplurality of subscriber stations by a TDMA system and one multipleaccess channel and one random access channel are assigned to one frameof the TDMA comprising the steps of: changing sector switchingprocedures in assignment of these channels based on a previously decidedpattern every TDMA super frame interval, and informing the subscriberstations of current information on the pattern in which sector switchingis changed via the multiple access channels.
 5. An interferenceavoidance method for a radio communication system in which multipleaccess is carried out between a base station which has a sectorswitching type antenna and a plurality of subscriber stations by a TDMAsystem and one multiple access channel and one random access channel areassigned to one frame of the TDMA system, comprising the steps of:changing sector switching procedures in assignment of these channelsbased on a previously decided pattern every TDMA super frame interval,and informing the subscriber stations of the sector switching proceduresafter sector switching is changed via the multiple access channels,which are selected prior to change of procedures by at least one frameinterval.
 6. A method for determining the existence of interference in aradio communication system in which multiple access is carried outbetween a base station and a plurality of subscriber stations using aTDMA transmission system wherein a plurality of random access channelsare assigned to one frame of the TDMA system, comprising the steps of:determining an interference detection rate in each of said plurality ofrandom access channels; comparing the determined interference detectionrates with each other; and determining the existence of interference ina particular random access channel when the determined interferencedetection rate of said particular random access channel is greater thandetermined interference detection rates in other random access channelsof said plurality of random access channels by a predetermined amount.7. The method according to claim 6, further comprising the steps of:detecting said interference from said plurality of random accesschannels for transmitting signals from said plurality of subscriberstations to the base station, closing the random access channel in whichinterference has been detected, changing arrangement of the randomaccess channel used by said subscriber station to another time slot, andinforming the subscriber station of the closing of the random accesschannel and time slot arrangement change information via multiple accesschannels.
 8. The method according to claim 7, further comprising thestep of monitoring the closed random access channel for a predeterminedtime to detect whether or not an interference wave is present.
 9. Amethod for determining the existence of interference in a radiocommunication system in which multiple access is carried out between abase station that has a sector switching type antenna for switchingbetween multiple sectors of a service area of said base station and aplurality of subscriber stations, using a TDMA transmission systemwherein a single random access channel is assigned to one frame of theTDMA system, comprising the steps of: determining an interferencedetection rate in each of a plurality of random access channels assignedto different frames; comparing the determined interference detectionrates with each other; and determining the existence of interference ina particular sector when the determined interference detection rate of arandom access channel in said particular sector is greater thandetermined interference detection rates in other random access channelsby a predetermined amount.
 10. The method according to claim 9, furthercomprising the steps of: detecting said interference from said pluralityof random access channels for transmitting signals from said pluralityof subscriber stations to the base station, closing the random accesschannel in which interference has been detected, changing arrangement ofthe random access channel used by said subscriber station to anothertime slot, and informing the subscriber station of the closing of therandom access channel and time slot arrangement change information viamultiple access channels.
 11. The method according to claim 10, furthercomprising the step of monitoring the closed random access channel for apredetermined time to detect whether or not an interference wave ispresent.